1. Field of the Invention
This invention relates to ironless rotor windings of drag cup type rotary coils used in miniature electric machines and to a method of making such ironless rotor windings.
2. Description of the Prior Art
In the rotary electric machines, and especially in the so-called micromotors, any slightest improvement in efficiency means great advantages in their uses or applications. In other words, any decrease in power loss may directly lead to the elimination of unstable factors, causes of heating, fluctuations and so forth.
The losses in motors include iron loss, copper loss and mechanical loss. If a rotor has no iron core, no hysteresis loss will occur due to the alternation of magnetic fluxes and the possible eddy current at the stator will be negligible, if any. Thus, it will be unnecessary to take the iron loss into consideration as a whole. Furthermore, the reactance voltage which may be produced in the coils during commutation will be extremely reduced due to the absence of the core, thereby ensuring an almost ideal commutation to take place and accordingly resulting in a stable operation and longer service of the commutators.
In the motors of the described type and having no iron loss, the following motor circuit formula is well satisfied: EQU I.sub.a V - I.sub.a.sup.2 R = I.sub.a E.sub.c ( 1)
where R = R.sub.a + R.sub.b and I.sub.a represents armature current, V; voltage of electric source, R.sub.a ; armature resistance, R.sub.b ; brush contact resistance, and E.sub.c : output voltage. Therefore, by designing the motors so that the copper loss I.sub.a.sup.2 R may be minimized relative to the input I.sub.a V, the output I.sub.a E.sub.c may be maximized. Thus, simply by minimizing the mechanical loss contained in the output I.sub.a E.sub.c, the motors may readily be increased in efficiency.
To reduce the copper loss, the armature coils must be formed of thicker conductors to decrease the electric resistance R. Such ironless rotor windings have heretofore been formed by two alternative methods. One of them is the method known as bobbinless winding or the method which employs no spool. According to this method, the rigidity of the conductor to be wound is utilized to wind it into successive close turns which are oblique as much as possible with respect to the axis of the winding, and usually one reciprocal winding cycle provides one layer of the conductor. In such a coil, its thickness in the air gap is twice the diameter of the wound conductor. The bobbinless winding method method often encounters difficulties in forming multi-layer coils because of its bobbinless nature, and this also limits the number of effective conductors.
The other known method employs a spool and permits the thickness of the coil in the air gap to be selected as desired in accordance with the size of the air gap. A wider air gap may be provided by forming a multilayer coil. In other words, the width of the air gap may be determined in accordance with the number of turns and thus, the number of effective conductors is not limited.
In the rotary electric machines of the drag cup type, the number of revolutions depends on the number of effective conductors in the coil. In order to provide a low-speed motor for a high torque, the number of effective conductors must be increased, In the first-named bobbinless winding method, however, the number of effective conductors is limited as described already, and an effort to increase such number would encounter the need to reduce the diameter of the conductors. As a result, the electric resistance R of the armature would be increased to thereby increase the copper loss and reduce the efficiency of the rotor as a whole.
Contrarily, the second-named spool-type winding method permits a conductor of greater diameter to be wound into multiple layers, which means adaptability of increasing number of effective conductors and maintaining a lower electric resistance, and thus results in a motor of higher efficiency.
Furthermore, motors manufactured by the first-named winding method may only be applicable for relatively high-speed rotation because of the limited number of conductors in their armatures, whereas motors manufactured by the second-named winding method permit the total number of conductors to be increased and therefor, irrespective of any increase in the size of the field air gap, the flux density in the air gap will be prevented from reduction by using less expensive magnets such as barium ferrite magnets instead of more expensive metallic magnets and utilizing the entire flux maintained by such magnets. Thus, the latter method leads to economical advantages of the motors resulting therefrom.